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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@Gordon
If you have an oscilloscope it'd be a lot easier to see what was happening...
I've just tried here and with two capacitors and two diodes I can get around 6v, which is about what you'd expect from the 3.3v output (minus some diode voltage drops).
Because of the way it works, increasing the frequency won't increase the voltage at all - it'll just increase the current, but only up to as much as the 20mA STM32 output can supply.